Piriformis is a small muscle deep in the buttock. It arises from the sacurm, and attaches to the proximal femur. The muscle usually causes problems when it is tight. This may happen due to a strain, or due to repeated micro trauma. Activities that cause it to stretch will produce pain.
Pirformis problems cause pain deep in the buttock. These pains are worst when the hip is rapidly rotated such as during rapid changes of direction when running. Pain my be felt at the extremes of motion.
If you think you have this injury it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any aggravating activities with may prolong the problem. It generally wont get better on its own. The first thing to do is reduce your activity and start gentle stretching.
Treatment involves first confirming the diagnosis then establishing the underlying cause. Options may include activity modification, physiotherapy, or anti inflammatories. Usually carefully directed physiotherapy is all that is needed.